The present invention relates to an acoustic impedance measuring apparatus for measuring the acoustic impedance of a target measurement object by using ultrasonic waves and, more particularly, to an acoustic impedance measuring apparatus for measuring the acoustic impedance from the surface to a deep portion of a target measurement object by using ultrasonic waves.
Conventionally, the research and development of a technology of measuring the physical properties of a vital tissue or of a complicated structure and displaying the measured physical properties in the form of an image have been done.
This technology is particularly important in medical diagnoses.
For example, when this technology is used to discriminate between a benign tumor and a malignant tumor and examine the necessity for a surgical operation, an unnecessary surgical operation can be avoided.
This makes it possible to avoid imposing physical and economical burdens on the patient.
Points presently recognized as problems or subjects of this technology are as follows.
For example, the discrimination between a benign cerebral aneurysm and a malignant cerebral aneurysm requires craniotomy because this discrimination is done by checking the color difference between them.
Also, it is known that many hematomas growing in deep portions of a liver are benign. However, it is in practice difficult to discriminate between a benign hematoma and a hardened cancer tissue, so a surgical operation is necessary for the sake of safety.
This is so because even when a conventional ultrasonic diagnostic apparatus observes a shadow, if the difference between the acoustic impedance of a hematoma and the acoustic impedance of a surrounding normal tissue is small, regardless of whether the former is higher or lower than the latter, similar echo images are displayed.
Also, it is necessary to increase the accuracy of measurement of the depth of a cancer and perform an optimum diagnosis not forcing any excessive burden on a patient.
As an effective solution for these problems and subjects, the development of an image diagnostic apparatus capable of accurately diagnosing the properties of a vital tissue has been desired.
Conventional ultrasonic diagnostic apparatuses use the direction of depth as the time axis of ultrasonic propagation and find the position on this time axis of a tissue whose acoustic impedance has a certain difference from the acoustic impedance of a peripheral tissue.
It is impossible for these apparatuses to find whether the acoustic impedance of an object is higher or lower than the acoustic impedance of a peripheral tissue.
Recently, however, to solve this problem of the conventional ultrasonic diagnostic apparatuses, the research of an ultrasonic diagnostic apparatus which estimates the elastic characteristics as properties of a vital tissue and displays an image of the elastic characteristics is being extensively sought.
For example, Japanese Patent No. 2629734 has disclosed a technology pertaining to an ultrasonic apparatus which measures the elastic characteristics of a target measurement object by applying ultrasonic waves and low-frequency vibrations to the object.
Referring to FIG. 22A, this ultrasonic diagnostic apparatus controls a burst signal (3.5 MHz) generator 204 and a power amplifier 205 to apply ultrasonic waves from an ultrasonic transducer array 202 to a sample 200 as a target measurement object via a gel substance 203.
A display device 212 detects the ultrasonic waves fed back from the sample 200 via the gel substance 203, the ultrasonic transducer array 202, an amplifier 206, a logarithmic amplifier 207, an envelope detector 208, a low-pass filter 209, an A/D converter 210, and a microcomputer 211.
In this measurement, a vibrator 216 which is supported by a spring balancer 213 and driven via a low-frequency oscillator 214 and a power amplifier 215 applies low-frequency vibrations to the sample 200 as a target measurement object via a vibrating plate 201. The vibrations propagate into the sample 200 as a target measurement object.
This propagating vibration pressure applies perturbation to a large number of fine reflectors inside the sample 200 as a target measurement object. The response from the sample 200 as a target measurement object when a reflector receives this perturbation is different from the response when no reflector receives the perturbation.
FIGS. 22B and 22C show the output waveforms of ultrasonic probe waves fed back from the sample 200 as a target measurement object and displayed on the display device 212 when ultrasonic waves are applied to the sample 200 with different low-frequency vibration phases.
FIG. 22D shows a waveform indicating the difference between FIGS. 22B and 22C.
On the basis of a change rate .DELTA.E shown in FIG. 22D, the susceptivity to perturbation of the interior of the sample 200 as a target measurement object, i.e., the elastic feature of the sample 200 can be known.
In FIG. 22A, reference numeral 217 denotes a timing signal generator for supplying necessary timing signals to the individual units described above.
Also, Jpn. Pat. Appln. KOKAI Publication No. 6-273396 describes an ultrasonic diagnostic apparatus which applies a drive pulse as an input signal as shown in FIG. 23A to an ultrasonic transducer and immediately detects the difference between waveform feature amounts, e.g., maximum amplitudes of an ultrasonic response signal 301 and an echo signal 302, thereby estimating the hardness of a target measurement object.
FIGS. 23C and 23D show FFT (Fast Fourier Transform) images of the ultrasonic response signal 301 and the echo signal 302, respectively, shown in FIG. 23B.
FIG. 24 shows a process flow of this ultrasonic diagnostic apparatus.
In this flow, after blank measurement (step S1), i.e., no-load measurement, a target measurement object is measured (step S2). After that, contact stress measurement (step S3), impulse response characteristic measurement (step S4), transmission signal waveform analysis (step S5), reception signal waveform analysis (step S6), and echo time analysis (step S7) are performed. The waveform feature amounts of the measurement results are compared by comparing the transmission/reception signal waveform analytical values and the blank measurement value (step S8). The comparison result and the echo time are divided into a time response characteristic and a nonlinear elastic value (step S9). Tactile perceptual data is formed from this result and the contact stress (step S10).
This ultrasonic diagnostic apparatus includes an ultrasonic transducer capable of detecting elastic characteristics, i.e., anisotropy in lateral and longitudinal directions, a means for generating a pulse voltage, a means for reading the amplitude and center frequency of a piezoelectric vibration signal excited by this pulse voltage, a means for reading the maximum amplitude and center frequency of an ultrasonic echo signal with respect to an ultrasonic transmission signal which is generated by the piezoelectric vibration signal and propagates in the direction of depth, and a perceptual data processing means for receiving these parameters and converting them into hardness data of a target measurement object.
Unfortunately, the technology disclosed in Japanese Patent No. 2629734 requires a vibrator for applying low-frequency vibrations to a target measurement object and propagating the vibrations inside the object. This vibrator is difficult to miniaturize because a vibrating mechanism is necessary and the vibrating end must be brought into contact with the object. This makes it difficult to apply this technology to an ultrasonic endoscopic system.
Also, in an extracorporeal ultrasonic diagnostic system, low-frequency vibrations are scattered by subcutaneous fat, ribs, and the body cavity. Therefore, it is uneasy to propagate such low-frequency vibrations from the body surface to a target vital tissue.
Additionally, the distribution of stress excited by low-frequency vibrations propagating in the direction of depth of a vital tissue must be uniform along a direction in which an ultrasonic signal propagates.
However, it is extremely difficult to keep constant the stress distribution generated in a vital tissue by propagation.
Furthermore, the presence of fine scattering bodies inside an object is a premise. However, fine scattering bodies are not necessarily present in an actual vital tissue. Also, even when scattering bodies exist, the level of an echo signal from such a scattering body is very low, so the signal may be buried in external noise or apparatus noise.
Accordingly, a special algorithm must be used to extract the echo signal buried in the noise level, and this makes real-time processing difficult.
Since the accuracy of diagnosis of the hardness of an object is insufficient due to the two latter restrictions, the aforementioned discrimination between a benign tumor and a malignant tumor is presumably difficult to perform in the initial stages of tumor growth.
Jpn. Pat. Appln. KOKAI Publication No. 6-273396 describes the technology of inputting the maximum amplitude and center frequency of an ultrasonic response signal (echo signal) as extraction parameters and converting them into hardness data of a target measurement object.
This Jpn. Pat. Appln. KOKAI Publication No. 6-273396, however, does not describe calculations of the characteristics concerning the elasticity of a target measurement object by using a predetermined algorithm.